This invention relates to Doppler radar, and more particularly to a method and apparatus for automatically classifying radar targets according to their distinctive target signatures. In the prior art, techniques for classifying such targets have involved splitting the Doppler frequency signals indicative of moving targets into two or more ranges corresponding to their radial velocities. This technique can discriminate between fast moving vehicles and men on foot, however it cannot distinguish between a man on foot and a slow moving vehicle with a radial velocity comparable to that of the moving human target.
For these reasons, discrimination between slow moving vehicles and men on foot has been accomplished by aural monitoring of battlefield Doppler signals. The present invention comprises a technique and circuitry which automatically provides this discrimination and thus is capable of utilizing equipment to perform a function which has hitherto required skilled military personnel. The technique involves sensing the distinctive step modulation of the Doppler echo signal of a moving human target. This step modulation comprises a frequency modulation of the Doppler signal of the moving man caused by body motion related to his step movements. It results from arm and leg movement as well as oscillatory torso movement due to the stepping movement. This so called step frequency is detected by means of a specially selected frequency modulation detector and circuitry is provided for splitting the step frequency signals into several bands corresponding to targets stepping at different speeds, for example, the lowest stepping frequency may correspond to a slowly walking man and the highest to a running man. The invention can be utilized with radars with or without range measuring capability.